Cardiac rhythm management system with maximum tracking rate (MTR) hysteresis

ABSTRACT

A cardiac rhythm management system provides both a safe maximum pacing rate limit and a physiological maximum pacing rate limit. In one embodiment, a normal maximum tracking rate (MTR) and a hysteresis MTR are provided. The hysteresis MTR is set higher than the normal MTR and functions as a maximum pacing rate. When an atrial rate exceeds the hysteresis MTR limit, the maximum pacing rate limit is set to the normal MTR. Once the atrial rate falls below a predetermined threshold, the maximum pacing rate limit is set to the hysteresis MTR. This provides for a more rapid and natural maximum pacing rate limit for a patient, while still protecting the patient from being paced at abnormally high rates.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a division of U.S. patent application Ser. No.10/012,887, filed on Nov. 6, 2001, now U.S. Pat. No. 6,904,316, thespecification of which is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to cardiac rhythm managementdevices, and more particularly, but not by way of limitation, to acardiac rhythm management system with maximum pacing rate hysteresis.

BACKGROUND INFORMATION

When functioning properly, the human heart maintains its own intrinsicrhythm, and is capable of pumping adequate blood throughout the body'scirculatory system. However, some people have abnormal cardiacelectrical conduction patterns and irregular cardiac rhythms that arereferred to as cardiac arrhythmias. Such arrhythmias result indiminished blood circulation. One mode of treatment includes use of acardiac rhythm management system. Such systems are often implanted in apatient and deliver electrical stimulation therapy to the patient'sheart.

Cardiac rhythm management systems include, among other things,pacemakers, also referred to as pacers. Pacemakers deliver timedsequences of low energy electrical stimuli, called pacing pulses, to theheart, typically via one or more intravascular leadwires or catheters(referred to as “leads”) each having one or more electrodes disposed inor about the heart. Heart contractions are initiated in response to suchpacing pulses (this is referred to as “capturing” the heart). Pacemakersalso sense electrical activity of the heart in order to detectdepolarization signals corresponding to the electrical excitationassociated with heart contractions. This function is referred to ascardiac sensing. Cardiac sensing is used to time the delivery of pacingpulses with the heart's intrinsic (native) rhythm. By properly timingthe delivery of pacing pulses, the heart can be induced to contract in aproper rhythm, greatly improving its output of blood. Pacemakers areoften used to treat patients with bradyarrhythmias (also referred to asbradycardias), that is, hearts that beat too slowly. For thatapplication, the pacemakers may operate in an “on-demand” mode, suchthat a pacing pulse is delivered to the heart only in absence of anormally timed intrinsic contraction. The on-demand pacing function isoften embodied in algorithms exhibiting pace inhibition, in which pacingin a lead is prevented (inhibited) for one heart beat when a cardiacdepolarization is detected in the same lead prior to the pace. Inbradycardia patients, for example, on-demand pacing can ensure thatpacing pulses are delivered only when the patient's intrinsic heart ratedrops below a predetermined minimum pacing rate limit, referred to as alower rate limit (LRL). Some pacemakers provide for two lower ratelimits, a first LRL, sometimes called a normal LRL, to provide a minimumnecessary heart rate during awake or exercise periods, and a second LRL,sometimes called a hysteresis LRL, to allow the heart to reach naturallyslower rates during sleep. When the patent's heart rate falls below thehysteresis LRL, the pacemaker switches to the normal LRL to ensure thepatient will have sufficient cardiac output by protecting the patientagainst abnormally slow heart rates.

Cardiac rhythm management systems also includecardioverters/defibrillators that are capable of delivering higherenergy electrical stimuli to the heart. Defibrillators are often used totreat patients with tachyarrhythmias (also referred to as tachycardias),that is, hearts that beat too quickly. Such too-fast heart rhythms alsocause diminished blood circulation because the heart is not allowedsufficient time to fill with blood before contracting to expel theblood. Such pumping by the heart is inefficient. A defibrillator iscapable of delivering an high energy electrical stimulus that issometimes referred to as a defibrillation countershock. The countershockinterrupts the tachyarrhythmia and allows the heart to reestablish anormal rhythm for efficient pumping of blood.

Cardiac rhythm management systems also include, among other things,pacemaker/defibrillators that combine the functions of pacemakers anddefibrillators, drug delivery devices, and any other implantable orexternal systems or devices for diagnosing or treating cardiacarrhythmias.

One problem faced by cardiac rhythm management systems is the treatmentof congestive heart failure (also referred to as “CHF”). CHF, which canresult from long-term hypertension, is a condition in which the musclein the walls of at least one of the right and left sides of the heartdeteriorates. By way of example, suppose the muscle in the walls of theleft side of the heart deteriorates. As a result, the left atrium andleft ventricle become enlarged, and the heart muscle displays lesscontractility, often associated with unsynchronized contractionpatterns. This decreases cardiac output of blood, and in turn, mayresult in an increased heart rate and less resting time between heartcontractions. This condition may be treated by conventional dual-chamberpacemakers and a new class of biventricular (or multisite) pacemakersthat are termed cardiac resynchronization therapy (CRT) devices. Aconventional dual-chamber pacemaker typically paces and senses oneatrial chamber and one ventricular chamber. A pacing pulse is timed tobe delivered to the ventricular chamber at the end of a programmedatrio-ventricular delay, referred to as AV delay, which is initiated bya pace delivered to or an intrinsic depolarization detected from theatrial chamber. This mode of pacing is sometimes referred to as anatrial tracking mode. The heart can be paced with a shortened AV delayto increase the resting time between heart contractions to increase theamount of blood that fills the ventricular chamber, thus increasing thecardiac output. Biventricular or other multisite CRT devices can paceand sense three or four chambers, usually including the right atrialchamber and both right and left ventricular chambers. By pacing bothright and left ventricular chambers, the CRT device can restore a moresynchronized contraction of the weakened heart muscle, thus increasingthe heart's efficiency as a pump. When treating CHF either withconventional dual-chamber pacemakers or CRT devices, it is critical topace the ventricular chambers continuously to shorten the AV delay or toprovide resynchronizing pacing, otherwise the patient will not receivethe intended therapeutic benefit. Thus the intention for treating CHFpatients with continuous pacing therapy is different from the intentionfor treating bradycardia patients with on-demand pacing therapy, whichis active only when the heart's intrinsic (native) rhythm is abnormallyslow.

Conventional pacemakers and CRT devices in current use rely onconventional on-demand pacing modes to deliver ventricular pacingtherapy. These devices need to be adapted to provide a continuous pacingtherapy required for treatment of CHF patients. One particular problemin these devices is that they prevent pacing when the heart rate risesabove a maximum pacing limit. One such maximum pacing limit is a maximumtracking rate (MTR) limit. “MTR” and “MTR interval,” where an “MTRinterval” refers to a time interval between two pacing pulses deliveredat the MTR, are used interchangeably, depending on convenience ofdescription, throughout this document. The MTR presents a problemparticularly for CHF patients, who typically have elevated heart ratesto maintain adequate cardiac output. When a pacemaker or CRT deviceoperates in an atrial tracking mode, it senses the heart's intrinsicrhythm that originates in the right atrial chamber, that is, theintrinsic atrial rate. As long as the intrinsic atrial rate is below theMTR, the device will pace one or both ventricular chambers after an AVdelay. If the intrinsic atrial rate rises above the MTR, the device willlimit the time interval between adjacent ventricular pacing pulses to aninterval corresponding to the MTR, that is, ventricular pacing rate willbe limited to the MTR. In this case, the heart's intrinsic contractionrate is faster than the maximum pacing rate allowed by the pacing deviceso that after a few beats, the heart will begin to excite the ventriclesintrinsically at the faster rate, which causes the device to inhibit theventricular pacing therapy due to the on-demand nature of its pacingalgorithm. The MTR is programmable in most conventional devices so thatthe MTR can be set above the maximum intrinsic atrial rate associatedwith the patient's maximum exercise level, that is, above thephysiological maximum atrial rate. However, many patients suffer fromperiods of pathologically fast atrial rhythms, called atrialtachyarrhythmia. Also some patients experience pacemaker-mediatedtachycardia (PMT), which occurs when ventricular pacing triggers anabnormal retrograde impulse back into the atrial chamber that is sensedby the pacing device and triggers another ventricular pacing pulse,creating a continuous cycle of pacing-induced tachycardia. During thesepathological and device-mediated abnormally elevated atrial rhythms, theMTR provides a protection against pacing the patient too fast, which cancause patient discomfort and adverse symptoms. Thus, to protect thepatient against abnormally fast pacing, the MTR often is programmed to alow, safe rate that is actually below the physiological maximum heartrate. For many CHF patients with elevated heart rates, this means thatthey cannot receive the intended pacing therapy during high butphysiologically normal heart rates, thus severely limiting the benefitof pacing therapy and the level of exercise they can attain. Therefore,there is a need for addressing this MTR-related problem in therapeuticdevices for CHF patients as well as other patients for whom pacingshould not be suspended during periods of fast but physiologicallynormal heart rates.

SUMMARY OF THE INVENTION

The present disclosure describes, among other things, a cardiac rhythmmanagement system and method providing both a safe maximum pacing ratelimit and a physiological maximum pacing rate limit. The present subjectmatter provides a solution to problems associated with the use of asingle maximum tracking rate (MTR). In one embodiment, the presentsubject matter utilizes two MTRs, where the first is a normal MTR andthe second is a hysteresis MTR. A heart rate is measured. In oneembodiment, the heart rate is measured from a ventricular location. Inan alternative embodiment, the heart rate is measured from an atriallocation. In one embodiment, the hysteresis MTR is set higher than thenormal MTR. The hysteresis MTR functions as a maximum pacing rate limituntil the heart rate exceeds the hysteresis MTR limit. When the heartrate exceeds the hysteresis MTR limit, the maximum pacing rate limit isset to the normal MTR. Once the heart rate falls below the normal MTR,the maximum pacing rate limit is set to the hysteresis MTR, oralternatively, once the heart rate falls below the hysteresis MTR, themaximum pacing rate limit is set to the hysteresis MTR.

By utilizing a hysteresis MTR, two or more MTRs are provided forhandling fast heart rates having either physiological or pathologicalorigins. The normal MTR prohibits pacing at a rate that may causeadverse effect on a patient. The hysteresis MTR, on the other hand,allows for uninterrupted pacing treatment for patients, such ascongestive heart failure (CHF) patients, who may display fast butphysiologically normal heart rates but still benefit from cardiacresynchronization therapy (CRT) at such fast heart rates. In otherwords, the present subject matter provides the patient with a treatmentusing a variable maximum pacing rate limit that is physiologically safeand beneficial to the patient.

In one embodiment, the hysteresis MTR interval is set equal to the sumof the programmed post-ventricular atrial refractory period (PVARP) plusthe atrio-ventricular pacing delay (AV delay). In another embodiment,the hysteresis MTR interval is set equal to the sum of the programmedPVARP plus the AV delay intervals plus a sensing window interval thatallows sufficient time for sensing and detecting an atrialdepolarization following after the PVARP expires.

In one embodiment, the PVARP is set to a first set value when themaximum ventricular pacing rate is set to the normal MTR, and is set toa second set value when the maximum ventricular pacing rate is set tothe hysteresis MTR. In an alternative embodiment, the PVARP is setdynamically as a function of a heart rate that is between a lower ratelimit (LRL) and the hysteresis MTR.

This summary is intended to provide an overview of the subject matter ofthe present patent application. It is not intended to provide anexhaustive or exclusive explanation of the invention. The detaileddescription is included to provide further information about the subjectmatter of the present patent application.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a timing schematic illustrating one embodiment of a pacingscheme utilizing a single MTR;

FIG. 2 is a schematic illustrating one embodiment of a cardiac rhythmmanagement device;

FIG. 3 is a flow chart illustrating one embodiment of the presentsubject matter;

FIG. 4 is a graph of a heart rate and a maximum pacing rate plotted asfunctions of time according to one embodiment of the present subjectmatter; and

FIG. 5 is a block diagram of one embodiment of a cardiac rhythmmanagement system according to the present subject matter.

DETAILED DESCRIPTION

In the following detailed description of the preferred embodiments,reference is made to the accompanying drawings that form a part hereof,and in which are shown by way of illustration specific embodiments inwhich the invention may be practiced. It is understood that otherembodiments may be utilized, and structural changes may be made withoutdeparting from the scope of the present subject matter.

The present subject matter is described in applications involvingimplantable cardiac rhythm management systems including, but not limitedto, pacemakers, cardioverter/defibrillators, pacemaker/defibrillators,and biventricular or other multi-site cardiac resynchronization therapy(CRT) devices. However, it is understood that the present methods andapparatus may be employed in unimplanted devices, including, but notlimited to, external pacemakers, cardioverter/defibrillators,pacemaker/defibrillators, biventricular or other multi-site CRT devices,monitors, programmers and recorders.

Referring now to FIG. 1, there is shown one embodiment in which aninterval between a sensed atrial depolarization and a subsequentventricular depolarization are extended through the use of a single MTR.At 100, an atrial depolarization is sensed. After a programmed AV delay104, a ventricular pacing pulse is delivered at 108. In one embodiment,the ventricular pacing pulses have a maximum limit of the single MTR.The atrial rate then increases at 110 (time interval between atrialdepolarizations decreases, leading to a rise in the atrial rate). Atthis point, the atrial rate has exceeded the MTR, but the MTR remains asthe limit for ventricular pacing. Thus, the time interval between atrialand ventricular depolarization, i.e., an actual AV delay, increases, asindicated at 120. On each subsequent cycle when the atrial rate exceedsthe MTR, the actual AV delay increases, as shown at 124 and 128. At 128,the actual AV delay becomes so great that an atrial depolarization 134is not sensed, as atrial depolarization 134 occurs during thepost-ventricular atrial refractory period (PVARP). Atrial depolarization134 results in an intrinsic ventricular depolarization 140 (i.e.,non-paced ventricular depolarization) which occurs prior to the MTRexpiring. As a result, CRT is lost and/or compromised when the atrialrate rises above the MTR.

The present subject matter provides a solution to this problem, amongother problems associated with the use of a single MTR. By utilizing asecond MTR, also referred to in this document as the hysteresis MTR, inaddition to a first MTR, also referred to in this document as the normalMTR, two or more MTRs are provided to deal with fast atrial rates (e.g.,atrial tachycardia rates) having either physiological or pathologicalorigins. In one embodiment, the hysteresis MTR is set higher than thenormal MTR. The hysteresis MTR is the maximum pacing rate limit untilthe heart rate exceeds the hysteresis MTR. The maximum pacing rate limitthen falls back to MTR when the heart rate exceeds the hysteresis MTR.Finally, the maximum pacing rate limit reverts to the hysteresis MTRwhen the heart rate falls below a predetermined threshold. In oneembodiment, the predetermined threshold is set to the normal MTR. In analternative embodiment, the predetermined threshold is set to thehysteresis MTR. Changing the maximum pacing rate limit in this fashionallows for an alternative treatment for patients, such as CHF patients,who display fast heart rates and need CRT at the faster heart rates.This alternative treatment provides for a more rapid and natural maximumpacing rate limit for the patient, but still protects the patient fromtracking abnormally high rates by limiting pacing to a lower maximumtracking rate when the heart rate exceeds the hysteresis MTR.

Referring now to FIG. 2, there is shown one embodiment of a cardiacrhythm management system 200. The cardiac rhythm management system 200includes an implantable cardiac rhythm management device 205, alsoreferred to as an implantable pulse generator, which is coupled to afirst intravascular endocardial lead 210 and a second intravascularendocardial lead 220. Additional leads may be connected to providemulti-site CRT. System 200 also includes an external programmer 225wirelessly communicating with device 205 using a telemetry device 230.

Each of leads 210 and 220 includes a proximal end 235 having a connectorportion which is coupled to device 205, and a distal end that can beimplanted into a heart to sense one or more cardiac signals. Each ofleads 210 and 220 includes one or more electrodes at the distal end forsensing cardiac signals and delivering electrical energy pulses (e.g.,pacing pulses).

In the embodiment shown in FIG. 2, lead 210 includes one or moreelectrodes, such as a tip electrode 240 and a ring electrode 244, forsensing cardiac signals and/or delivering pacing therapy. Lead 210 canalso include additional electrodes, such as defibrillation electrodesfor delivering atrial and/or ventricular cardioversion/defibrillationand/or pacing therapy. Lead 220 includes one or more electrodes, such asa tip electrode 250 and a ring electrode 254, for sensing cardiacsignals and/or delivering pacing therapy. Lead 220 optionally alsoincludes additional electrodes, such as defibrillation electrodes fordelivering atrial and/or ventricular cardioversion/defibrillation and/orpacing therapy.

Device 205 includes components enclosed in a hermetically-sealed housing256. Housing 256 can be used as an additionalpacing/cardioversion/defibrillation electrode. More generally, thepresent subject matter works with a variety of other electrodeconfigurations and with a variety of electrical contacts or“electrodes”, as are known in the art.

Referring now to FIG. 3, there is shown one embodiment of a methodaccording to the present subject matter. Once started at 300, a cardiacsignal is sensed. At 310, a heart rate is determined from cardiacdepolarizations detected in the cardiac signal. In one embodiment, thecardiac signal is sensed from a ventricular location, and the heart rateis determined based on a time interval between two adjacent ventriculardepolarizations (R-waves), referred to as an R-R interval or ventricularcycle length. In another embodiment, the heart rate is determined basedon an R-R interval value averaged over a predetermined number ofventricular cycles. In an alternative embodiment, the cardiac signal issensed from an atrial location, the heart rate is determined based on atime interval between two adjacent atrial depolarizations (P-waves),referred to as a P-P interval or an atrial cycle length. In anotheralternative embodiment, the heart rate is determined based on a P-Pinterval value averaged over a predetermined number of atrial cycles. At320, the heart rate is compared to a first predetermined value and asecond predetermined value, where the second predetermined value isgreater than the first predetermined value. According to one embodimentof the present subject matter, the first predetermined value is a normalMTR and the second predetermined value is a hysteresis MTR, aspreviously discussed.

At 330, the system determines whether the heart rate exceeds the secondpredetermined value and whether the heart rate falls below apredetermined threshold. When the heart rate exceeds the secondpredetermined value, a maximum ventricular pacing rate is set to thefirst predetermined value, at 340. If, however, at 330 the systemdetects that the heart rate falls below the predetermined threshold, thesystem sets the maximum ventricular pacing rate to the secondpredetermined value at 350. In one embodiment, the predeterminedthreshold is set to the first predetermined value. In an alternativeembodiment, the predetermined threshold is set to the secondpredetermined value. In an additional embodiment, at 340, when the heartrate exceeds the second predetermined value, the maximum ventricularpacing rate is set to the first predetermined value, and the change ofthe maximum ventricular pacing rate from the second predetermined valueto the first predetermined value is done gradually over a predeterminedtime interval (i.e., the switch from the hysteresis MTR to the normalMTR is rate-smoothed). The system then returns to 300 to repeat theanalysis just described.

In one embodiment, the present subject matter is used in a tracking modeof pacing, in which a maximum pacing rate limit is required. During thetracking mode, an intrinsic depolarization sensed at a first heart sitetriggers a pace at a second heart site with or without a pre-determineddelay. In one embodiment, the tracking mode is an atrial tracking mode,in which the first heart site is in an atrial chamber, typically on theright side of the heart. One example of the atrial tracking mode is aDDD mode, in which a cardiac rhythm management system senses anintrinsic depolarization in the atrial chamber and triggers a pace inthe ventricular chamber after a predetermined AV delay. The DDD mode isalso an example of a tracking mode combined with on-demand pacing. Forexample, in the DDD mode, an intrinsic depolarization sensed in theatrial chamber may be followed by an intrinsic depolarization sensed inthe ventricular chamber after an intrinsic AV interval (a time intervalbetween an intrinsic atrial depolarization and an adjacently subsequentintrinsic ventricular depolarization) that is shorter than the AV delay.When this occurs, the intrinsic depolarization sensed in the ventricleinhibits the ventricular pace that would occur at the end of the AVdelay. In tracking modes, the pacing rate increases in response to theintrinsic heart rate. As long as the heart's intrinsic rhythm isphysiologically normal, these modes of operation are safe, but when theintrinsic rhythm becomes abnormally fast, such as during an atrialtachyarrhythmia, tracking might increase the pacing rate tonon-physiological and harmful levels. To prevent such occurrences, atracking mode of operation traditionally provides for a maximum pacingrate limit, the MTR, which limits the time interval between pacingpulses to be no faster than the MTR interval. Tracking modes provideadditional protection against pacemaker-mediated tachycardia (PMT),which occurs when a pacing pulse delivered to the second heart sitecauses a detectable depolarization at the first heart site that triggersanother pacing pulse to be delivered to the second heart site again,which continues in a loop. PMT in tracking modes can be prevented byusing a post-ventricular atrial refractory period (PVARP), which is aperiod of time following the pace at the second heart site during whichsensing is blocked or ignored (i.e., made refractory) at the first heartsite. Also, tracking modes provide for stopping a PMT once it occurs byincreasing the duration of the PVARP, called PVARP extension, when a PMTis detected.

CHF patients may benefit from continuous atrial-synchronous ventricularpacing that operates in an atrial tracking mode. The benefit arises fromthe coordination of the atria and ventricles, and not simply as ratesupport for the ventricles. Typically, when the atrial rate exceeds theMTR, ventricular pacing is inhibited or non-optimal. Examples ofinhibited ventricular pacing include where an intrinsic AV intervalresulting from the fast atrial rate causes a ventricular depolarizationbefore the end of the MTR interval, which in turn would cause theventricular pacing to be inhibited. Additionally, it is possible that bylimiting the pacing rate to the MTR, the duration between the atrial andventricular contractions will be extended, which may lead to non-optimalatrio-ventricular synchrony. To solve this problem, in one embodiment,the present subject matter provides a tracking mode with two MTRs, wherethe first is a normal MTR and the second is a hysteresis MTR. The valuesfor the normal MTR and the hysteresis MTR are programmable in thecardiac rhythm management system. In one embodiment, the hysteresis MTRis set higher than the normal MTR. The hysteresis MTR functions as themaximum pacing rate limit while tracking a heart rate until the heartrate exceeds the hysteresis MTR limit. When the heart exceeds thehysteresis MTR limit, the maximum pacing rate limit is set to the normalMTR. Once the heart rate falls below a predetermined threshold, themaximum pacing rate limit is set to the hysteresis MTR. In oneembodiment, the predetermined threshold is set to the normal MTR. In analternative embodiment, the predetermined threshold is set to thehysteresis MTR. Thus, the present subject matter provides a trackingmode with, among other things, a safe maximum pacing rate limit and aphysiological maximum pacing rate limit.

In another embodiment, the present subject matter is used in a triggeredmode of pacing, in which a maximum pacing rate limit is also required.In a triggered mode, an intrinsic depolarization sensed at a heart sitetriggers one or more pacing pulses to be delivered to that site, and/orto one or more different heart sites, with or without a pre-determineddelay. One example of a triggered mode is a VVT mode, in which thecardiac rhythm management system senses an intrinsic depolarization in aventricular chamber and triggers an immediate pace in the sameventricular chamber. Another example of a triggered mode is an AAT mode,in which the cardiac rhythm management system senses an intrinsicdepolarization in an atrial chamber and triggers an immediate pace inthe same atrial chamber. Still another example of a triggered mode is abiventricular trigger mode, in which the cardiac rhythm managementsystem senses an intrinsic depolarization in a ventricular site andtriggers one or more pacing pulses to be delivered, with or without apredetermined delay, to the same ventricular site, or to one or moredifferent ventricular sites, or to the same ventricular site and one ormore different ventricular sites. During triggered modes, a heartchamber is paced at the intrinsic depolarization rate of the chamber. Ifthat intrinsic rate exceeds a maximum pacing rate limit, the triggeredpacing of that chamber is inhibited as long as the intrinsic rateremains above the maximum pacing rate limit. For triggered modes, thepresent subject matter provides a second, hysteresis maximum pacing ratelimit with exactly the same behavior as the hysteresis MTR describedpreviously for tracking modes. In one embodiment, the present subjectmatter provides a triggered mode with two maximum pacing rates, wherethe first is a normal maximum pacing rate and the second is a hysteresismaximum pacing rate. The values for the normal maximum pacing rate andthe hysteresis maximum pacing rate are programmable in the cardiacrhythm management system. In one embodiment, the hysteresis MTR is sethigher than the normal MTR. The hysteresis maximum pacing rate functionsas the effective maximum pacing rate limit while pacing being triggeredby intrinsic depolarizations until the intrinsic heart rate exceeds thehysteresis maximum pacing rate. When the intrinsic heart rate exceedsthe hysteresis maximum pacing rate, the effective maximum pacing ratelimit is set to the normal maximum pacing rate. Once the intrinsic heartrate falls below a predetermined threshold, the maximum pacing ratelimit is set to the hysteresis maximum pacing rate. In one embodiment,the predetermined threshold is set to the normal maximum pacing rate. Inan alternative embodiment, the predetermined threshold is set to thehysteresis maximum pacing rate.

In still another embodiment, the present subject matter is used in arate smoothing mode of pacing, in which a maximum pacing rate limit isalso required. The rate smoothing mode operates by adjusting a pacingrate based on a previous heart rate. In one embodiment, the cardiacrhythm management system operating in the rate smoothing mode limits thevariation of the pacing rate to a predetermined percentage of theprevious heart rate (intrinsic or paced). In one embodiment, a ratesmoothing mode is combined with a tracking mode to provide on-demandpacing. Both the tracking mode and rate smoothing mode would increasethe pacing rate in response to a heart's intrinsic rhythm. As long asthe heart's intrinsic rhythm is physiologically normal, these modes ofoperation are safe, but when the intrinsic rhythm becomes abnormallyfast, such as during an atrial tachyarrhythmia, tracking and ratesmoothing modes might increase the pacing rate to non-physiological andharmful levels. To prevent the pacing rate from becoming too fast whenthe intrinsic heart rate is increasing, a maximum pacing rate isprovided so that when the previous heart rate would require pacing at arate above the maximum pacing rate, the pacing is inhibited as long asthe indicated pacing rate remains above the maximum pacing rate. Forrate smoothing modes, the present subject matter provides a second,hysteresis maximum pacing rate limit with exactly the same behavior asthe hysteresis maximum pacing rate described previously for triggeringmodes.

In one embodiment, the first predetermined value has a value in therange from 50 to 180 beats/minute, where 120 beats/minute is a possiblevalue. The second predetermined value has a value in the range from 50to 180 beats/minute, where 140 beats/minute is a possible value. In analternative embodiment, the second predetermined value (e.g., thehysteresis MTR) differs from the first predetermined value (e.g., thenormal MTR) by a predetermined offset. In one embodiment, thepredetermined offset is a programmable value in the range of 10 to 100beats/minute, where 20 beats/minute is a possible value. In anotherembodiment, the second predetermined value (e.g., the hysteresis MTR)corresponds to the sum of a programmed PVARP plus an AV delay (e.g., thehysteresis MTR interval equals to the sum of the PVARP plus the AVdelay). In still another embodiment, the second predetermined value(e.g., the hysteresis MTR) corresponds to the sum of the programmedPVARP plus the AV delay intervals plus a sensing window interval (e.g.,the hysteresis MTR interval equals to the sum of the PVARP plus the AVdelay plus the sensing window). In one embodiment, the sensing windowinterval is a programmable value in the range of 0 to 1000 milliseconds,where 200 milliseconds is a possible value. In another embodiment, thesensing window interval is a percentage of the sum of the PVARP plus theAV delay, where the percentage is a programmable value in the range of 0to 90%, where 50% is a possible value. In a further embodiment, thepredetermined time interval over which the maximum ventricular pacingrate is changed from the second predetermined value to the firstpredetermined value has a value in the range from 0 to 120 seconds,where 10 seconds is a possible value.

In one embodiment, the PVARP is a programmable value, which can be setbased on the first predetermined value and the second predeterminedvalue (i.e., the first predetermined value and second predeterminedvalue can be associated with different PVARP settings to provide anadequate atrial sensing time for each rate range). In one embodiment,the PVARP is set to a first set value when the maximum ventricularpacing rate is set to the first predetermined value, and is set to asecond set value when the maximum ventricular pacing rate is set to thesecond predetermined value. In an alternative embodiment, the PVARP isset dynamically as a function of the heart rate between a lower ratelimit (LRL) and the hysteresis MTR (second predetermined value). In onesuch embodiment, the PVARP is set to a first set value at the LRL, andis set to a second set value at the hysteresis MTR, and is set to avalue between the first and second set values, in correspondence withdistances of the determined heart rate from the LRL and the hysteresisMTR. In another alternative embodiment, the PVARP associated with eachMTR can be set such that the sum of the PVARP plus the AV delay is equalto a predetermined MTR interval minus the sensing window interval. Inone embodiment, the sensing window interval is a programmable value inthe range of 0 to 1000 milliseconds, where 200 milliseconds is apossible value. In an alternative embodiment, the sensing windowinterval is a percentage of the predetermined MTR interval, where thepercentage is a programmable value in the range of 0 to 90%, where 50%is a possible value.

In an additional embodiment, the PVARP having the first set value isextended for at least one cardiac cycle length (a time interval betweentwo consecutive heart contractions sensed from the same lead) aftersetting the maximum ventricular pacing rate to the first predeterminedvalue when the heart rate exceeds the second predetermined value. In oneembodiment, this is done to protect against sensing retrograde atrialevents that can lead to a pacemaker-mediated tachycardia (PMT).

In another additional embodiment, PMT can be interrupted by ignoring theatrial sense and not triggering the AV delay for one cardiac cycle everyN cycles when the heart rate is above the first predetermined value. Inone embodiment, the atrial sense is ignored by extending the PVARP. Inanother embodiment, the value of N is programmable in the range of 10 to100, where 25 is a value that can be used.

In still another additional embodiment, atrial depolarizations sensedduring the PVARP can cause the maximum ventricular pacing rate to be setto a different value. For example, when an atrial depolarization isdetected during the PVARP, the maximum ventricular pacing rate is set tothe first predetermined value. This additional embodiment may be addedto the present subject matter to allow for the maximum ventricularpacing rate to be changed from the second predetermined value to thefirst predetermined value. In an additional embodiment, the switch fromthe second predetermined value to the first predetermined value is alsotriggered by a ventricular pacing rate that is equal to the secondpredetermined value, so that when ventricular pacing pulses aredelivered at a rate equal to the second predetermined value, the maximumventricular pacing rate is set to the first predetermined value.

Referring now to FIG. 4, there is shown one embodiment of a heart rate400 plotted as a function of time 404. In addition, there is shown twolines representing the first predetermined value 408 and the secondpredetermined value 410. In one embodiment, the first predeterminedvalue 408 represents the normal MTR, and the second predetermined value410 represents the hysteresis MTR. A curve 414 represents a heart rateas a function of time. The heart rate may be measured from a ventricularchamber or, alternatively, an atrial chamber. A line 442 represents themaximum pacing rate as a function of time. Heart rate 414 starts with aninitial value 420. In one embodiment, the maximum pacing rate isinitially set to the second predetermined value 410.

As heart rate 414 is determined, the value of heart rate 414 is comparedto the first predetermined value 408 and the second predetermined value410, as previously described. As shown in FIG. 4, heart rate 414 beginsto increase from initial value 420 as time progresses. At 424, heartrate 414 exceeds the first predetermined value 408. At 428, heart rate414 exceeds the second predetermined value 410. In one embodiment, onceheart rate 414 exceeds the second predetermined value 410, at 428,maximum ventricular pacing rate 442 is set to the first predeterminedvalue 408. Heart rate 414 is then shown to decrease (i.e., drop). Whenheart rate 414 falls below a predetermined threshold, maximumventricular pacing rate 442 is set to the second predetermined value410. In one embodiment (as shown in FIG. 4), when heart rate 414 fallsbelow the first predetermined value 408 at 440, maximum ventricularpacing rate 442 is set to the second predetermined value 410. In analternative embodiment (not shown in FIG. 4), when heart rate 414 fallsbelow the second predetermined value 410, maximum ventricular pacingrate 442 is set to the second predetermined value 410.

Referring now to FIG. 5, there is shown a schematic diagram of a cardiacrhythm management system 500 according to one embodiment of the presentsubject matter. In one embodiment, system 500 includes circuitry whichreceives one or more cardiac signals and delivers electrical energy toelectrodes positioned within a patient's heart. System 500 includes afirst sensing circuit 504, a second sensing circuit 508, a controller502, a therapy circuit 514, and a power source 512. In one embodiment,controller 502 is a microprocessor-based system, including a ratedetector 520, a comparator 510, a pacing algorithm module 522, and amemory circuit 524. Memory circuit 524 contains parameters for variouspacing and sensing modes and stores data indicative of cardiac signalsreceived by the controller 502. In one embodiment, system 500 receivescardiac signals through connection terminals 530, 532, 534 and 536,which are coupled to the electrodes attached to intravascularendocardial leads (e.g., the first and second intravascular endocardialleads, as previously described).

A hermetically sealed housing 540 encases the implantable circuitry ofsystem 500. Housing 540 is suitable for implantation in a human body. Inone embodiment, the housing 540 is made of titanium. However, otherbiocompatible housing materials as are known in the art may also beused. A connector module 544, referred to as a header, is attached tohousing 540 to allow for physical and electrical connections between theintravascular endocardial leads and the encased circuitry.

First sensing circuit 504 includes a first sense amplifier 550 and afirst cardiac depolarization detector 552. Second sensing circuit 508includes a second sense amplifier 554 and a second cardiacdepolarization detector 556. First sensing circuit 504 is electricallycoupled to terminals 530 and 532. Second sensing circuit 508 iselectrically coupled to terminals 534 and 536. These terminals, in turn,are coupled to heart sites, via intravascular endocardial leads, toallow for the one or more cardiac signals to be sensed in the heart. Inthe present embodiment, first sensing circuit 504 receives a firstcardiac signal, and second sensing circuit 508 receives a second cardiacsignal. In one embodiment, one or more cardiac signals are sensed froman atrium and amplified. Atrial depolarizations, including P-waves, aredetected and routed to controller 502. In one embodiment, one or morecardiac signals are sensed from one or more ventricles and amplified.Ventricular depolarizations, including R-waves, are detected and routedto controller 502. In one embodiment, one or more cardiac signals aresensed from an atrium and one or more ventricles and amplified. Atrialand ventricular depolarizations, including P-waves and R-waves, aredetected and routed to controller 502.

Pacing algorithm module 522 responds to, among other things, sensingcircuits 504 and 508 by providing pacing commands to therapy circuit514, as needed according to the programmed pacing mode. In oneembodiment, therapy circuit 514 delivers pacing pulses to one or moreintravascular endocardial leads. Power to system 500 is supplied bypower source 512. In one embodiment, power source 512 includes one ormore electrochemical batteries.

In one embodiment, rate detector 520 determines a heart rate based onthe cardiac depolarizations detected by depolarization detector 552 ordepolarization detector 556. In one embodiment, rate detector 520determines an atrial rate as the heart rate. In an alternativeembodiment, rate detector 520 determines a ventricular rate as the heartrate.

Comparison circuit 510 receives the heart rate and compares the heartrate to the first predetermined value and the second predeterminedvalue. The result of the comparison is routed to pacing algorithm module522, which sets the maximum ventricular pacing rate to the firstpredetermined value when the heart rate exceeds the second predeterminedvalue and sets the maximum ventricular pacing rate to the secondpredetermined value when the heart rate falls below a predeterminedthreshold, as previously described. In one embodiment, the predeterminedthreshold is set to the first predetermined value. In an alternativeembodiment, the predetermined threshold is set to the secondpredetermined value. In a further embodiment, when pacing algorithmmodule 522 changes the maximum ventricular pacing rate from the secondpredetermined value to the first predetermined value, it does so over apredetermined time interval when the heart rate exceeds the secondpredetermined value.

In an additional embodiment, pacing algorithm module 522 sets the PVARPbased on the first predetermined value and the second predeterminedvalue. In one embodiment, pacing algorithm module 522 sets the PVARP toa first set value when the maximum ventricular pacing rate is set to thefirst predetermined value, and sets the PVARP to a second set value whenthe maximum ventricular pacing rate is set to the second predeterminedvalue. In one embodiment, the PVARP is set to a first set value at theLRL, and is set to a second set value at the hysteresis MTR, and is setto a value between the first and second set values in correspondencewith the distances of the determined heart rate from the LRL and thehysteresis MTR, as previously described. In an alternative embodiment,the PVARP associated with each MTR can be set such that the sum of thePVARP and the AV delay is equal to the predetermined MTR interval minusa programmable sensing window interval, as previously described. Inanother alternative embodiment, controller 502 sets the maximumventricular pacing rate to the first predetermined value when an atrialdepolarization is detected during the PVARP. In still anotheralternative embodiment, controller 502 sets the maximum ventricularpacing rate to the first predetermined value when atrial depolarizationsare detected during the respective PVARPs over a predetermined number ofconsecutive cardiac cycles. In yet another alternative embodiment,controller 502 sets the maximum ventricular pacing rate to the firstpredetermined value when atrial depolarizations are detected during therespective PVARPs in at least a predetermined number of cardiac cyclesover a predetermined total number of consecutive cardiac cycles.

In an additional embodiment, pacing algorithm module 522 extends thePVARP having the first set value for at least one cardiac cycle lengthafter setting the maximum ventricular pacing rate to the firstpredetermined value when the heart rate exceeds the second predeterminedvalue to protect against sensing retrograde atrial events. In a furtherembodiment, controller 502 ignores the atrial depolarizations and doesnot trigger the AV delay for one cardiac cycle every N cardiac cycleswhen the heart rate is above the first predetermined value. This assistsin stopping PMT, as previously described. In an additional embodiment,when therapy circuit 514 delivers ventricular pacing pulses at thesecond maximum pacing rate, pacing algorithm module 522 sets the maximumventricular pacing rate to the first predetermined value.

In one embodiment, a communication circuitry 570 is additionally coupledto controller 502 to allow system 500 to communicate with an externalprogrammer 576. In one embodiment, communication circuitry 570 includesa data receiver and a data transmitter to receive and transmit signals,including cardiac data, to and from external programmer 576. In oneembodiment, the data receiver and the data transmitter include a wireloop antenna to establish a radio frequency telemetry link, as is knownin the art, to receive and transmit signals and data to and fromexternal programmer 576.

It is understood that the above description is intended to beillustrative, and not restrictive. Many other embodiments will beapparent to those of skill in the art upon reviewing the abovedescription. The scope of the invention should therefore, be determinedwith reference to the appended claims, along with the full scope ofequivalents to which such claims are entitled.

1. A method, including: sensing a first cardiac signal from an atriallocation; detecting atrial depolarizations from the first cardiacsignal; determining an atrial rate based on the atrial depolarizations;comparing the atrial rate to a predetermined threshold and a secondpredetermined value; setting a maximum ventricular pacing rate in acardiac rhythm management device using a result of the comparing, thesetting including: setting the maximum ventricular pacing rate to afirst predetermined value when the atrial rate exceeds the secondpredetermined value, the second predetermined value larger than thefirst predetermined value; and setting the maximum ventricular pacingrate to the second predetermined value when the atrial rate falls belowthe predetermined threshold.
 2. The method of claim 1, wherein themaximum ventricular pacing rate is a maximum tracking rate (MTR).
 3. Themethod of claim 1, wherein the predetermined threshold is set to thefirst predetermined value.
 4. The method of claim 1, wherein thepredetermined threshold is set to the second predetermined value.
 5. Themethod of claim 1, wherein setting the maximum ventricular pacing rateto the first predetermined value when the atrial rate exceeds the secondpredetermined value includes changing the maximum ventricular pacingrate from the second predetermined value to the first predeterminedvalue over a predetermined time interval.
 6. The method of claim 1,further including: setting the PVARP to a first set value when themaximum ventricular pacing rate is set to the first predetermined value;and setting the PVARP to a second set value when the maximum ventricularpacing rate is set to the second predetermined value.
 7. The method ofclaim 6, further including: calculating the first predetermined valuebased on at least the first set value and an atrioventricular pacingdelay (AV delay); and calculating the second predetermined value basedon at least the second set value and the AV delay.
 8. The method ofclaim 6, further including: calculating the first predetermined valuebased on at least the first set value, an AV delay, and a first sensingwindow interval; and calculating the second predetermined value based onat least the second set value, the AV delay, and a second sensing windowinterval.
 9. The method of claim 8, wherein: the first sensing windowinterval is a predetermined percentage of a sum of the first set valueand the AV delay; and the second sensing window interval is apredetermined percentage of a sum of the second set value and the AVdelay.
 10. The method of claim 8, wherein the sensing window interval isa programmable value being a function of at least one of the first andsecond predetermined values.
 11. The method of claim 6, furtherincluding extending the PVARP associated with the first predeterminedvalue for at least one cardiac cycle length after setting the maximumventricular pacing rate to the first predetermined value when the atrialrate exceeds the second predetermined value.
 12. The method of claim 6,further including calculating the second predetermined value from thePVARP and an AV delay.
 13. The method of claim 1, further includingignoring any atrial depolarization and not triggering an AV delay forone cardiac cycle every N cardiac cycles when the atrial rate exceedsthe first predetermined value.
 14. The method of claim 1, furtherincluding setting the maximum ventricular pacing rate to the firstpredetermined value when an atrial depolarization is sensed during apostventricular atrial refractory period (PVARP).
 15. The method ofclaim 1, further including setting the maximum ventricular pacing rateto the first predetermined value when at least a predetermined number ofatrial depolarizations are sensed during postventricular atrialrefractory periods (PVARPs) over a predetermined number of consecutivecardiac cycles.
 16. The method of claim 1, further including: deliveringventricular pacing pulses; and setting the maximum ventricular pacingrate to the first predetermined value when the ventricular pacing pulsesare delivered at a rate equal to the second predetermined value.
 17. Amethod, including: sensing a first cardiac signal from a ventricularlocation; detecting ventricular depolarizations from the first cardiacsignal; determining a heart rate based on the ventriculardepolarizations; comparing the heart rate to a first predetermined valueand a second predetermined value, the second value larger than the firstvalue; setting a maximum atrial pacing rate in a cardiac rhythmmanagement device to the first predetermined value when the heart rateexceeds the second predetermined value; and setting the maximum atrialpacing rate in a cardiac rhythm management device to the secondpredetermined value when the heart rate falls below a predeterminedthreshold.
 18. The method of claim 17, wherein the predeterminedthreshold is set to the first predetermined value.
 19. The method ofclaim 17, wherein the predetermined threshold is set to the secondpredetermined value.
 20. A method, including: sensing a first cardiacsignal from an atrial location; detecting atrial depolarizations fromthe first cardiac signal; determining a heart rate based on the atrialdepolarizations; comparing the heart rate to a first predetermined valueand a second predetermined value, the second value larger than the firstvalue; setting a maximum atrial pacing rate in a cardiac rhythmmanagement device to the first predetermined value when the heart rateexceeds the second predetermined value; and setting the maximum atrialpacing rate in a cardiac rhythm management device to the secondpredetermined value when the heart rate falls below a predeterminedthreshold.
 21. The method of claim 20, wherein the predeterminedthreshold is set to the first predetermined value.
 22. The method ofclaim 20, wherein the predetermined threshold is set to the secondpredetermined value.